As is well-understood in the art, a lithographic process includes the patterned exposure of a resist so that portions of the resist can be selectively removed to expose underlying areas for selective processing such as by etching, material deposition, implantation and the like. Traditional lithographic processes utilize electromagnetic energy in the form of ultraviolet light for selective exposure of the resist. As an alternative to electromagnetic energy (including x-rays), charged particle beams have been used for high resolution lithographic resist exposure. In particular, electron beams have been used to produce accurately controllable patterns for many uses, but find particular utility in mask making. Electron beam lithographic systems may be categorized as electron-beam probe focused lithography systems and electron beam projection lithography systems.
In scanning-type lithography (one example of a probe system), the substrate is sequentially exposed by means of a focused electron beam, wherein the beam either scans in the form of lines over the whole specimen and the desired structure is written on the object by corresponding blanking of the beam, or, as in a vector scan method, the focused electron beam is guided over the regions to be exposed. The beam spot may be shaped by a diaphragm. Scanning e-beam lithography is distinguished by high flexibility, since the circuit geometries are stored in the computer and can be optionally varied. Furthermore, very high resolutions can be attained by electron beam writing, since electron foci with small diameters may be attained with electron-optical imaging systems. However, it is disadvantageous in that the process is very time-consuming, due to the sequential, point-wise writing. Scanning e-beam lithography is therefore at present mainly used for the production of the masks used in projection lithography.
Recently, advances in electron optical systems have uncovered some newer approaches useful for performing electron beam lithography. One new approach is referred to as reflected electron beam lithography (REBL). An example of this approach is disclosed in detail in U.S. Pat. No. 6,870,172 to Mankos et al., entitled “Maskless Reflection Electron Beam Projection Lithography”.
FIG. 1 illustrates schematically how this approach works. An electron source (for example a thermionic emitter, or some other suitable electron emitter) 101 produces a beam of electrons 102 at a bias of 50 kV. The beam of electrons 102 is directed through illumination “optics” configured as electron-optics 104 for receiving and collimating the electron beam 102 from the source 101. Commonly, the illumination optics 104 require an arrangement of magnetic and/or electrostatic lenses configured to focus the electrons into electron beam 102 that is directed into a “magnetic prism” 106 that redirects the electron beam through objective optics 110 onto a electron beam pattern selector 112.
The magnetic prism 106 is a structure for deflecting the electron beam 102 in a direction perpendicular to its initial trajectory so that it is bent towards the objective lens 110 and the electron beam pattern selector 112. Commonly this is effectuated by using magnetic fields (obtained with magnetized plates, specialized windings, and pole pieces and the like) arranged to deviate the electron beam in the desired direction. Unfortunately, such magnetic prisms force electron beams along electron paths that can be of on the order of a meter or more in length. Such long path lengths are capable of seriously degrading the electron beam 102 and are not desirable.
The objective optics 110 generally include magnetic or electrostatic elements configured to decelerate electrons of the beam as they approach the electron beam pattern generator 112. The electron beam 102 is directed onto the electron beam pattern generator 112 that is configured to include an array of addressable elements or contacts. The array generally comprises an array of dynamically addressable metal contacts. This array can comprise an array of several million contacts if desired. A voltage level is controllably applied to the contacts to selectively reflect the electrons of the electron beam. For example, in areas of the pattern where no electrons are required, a positive bias can be applied to the metal contacts to absorb electrons and a negative bias can be applied to “reflect” the electrons away from selected contacts of the selector 112. Thus, by controlling the a pattern of voltages across the contacts of the electron beam pattern generator 112, the pattern of the reflected electron beam 113 can also be controlled.
As the reflected electrons 113 leave the selector 112, the objective optics accelerate the reflected electrons 113 toward their second pass through the prism 106. The prism 106 bends path of the reflected electrons 113 towards the projection optics 114.
The projection electron-optics 114 reside between the prism 106 and the target 116 (typically mounted on a movable stage). The projection optics 114 are typically configured to demagnify the beam 113 and focus the electron beam 113 onto a photoresist layer of a target (e.g., a wafer or mask). In this fashion, a desired pattern can be transferred onto the target (e.g., a layer of photoresist).
Although such processes and tools are suitable for their intended purposes, improvements can be made. The present invention seeks to go beyond the limitations and structural shortcomings of this existing technology.